Most electrical contacts are copper or copper alloy conductors with a tin-plated surface layer. The tin surface layer is a single continuous layer directly bonded to a clean non-oxidized copper substrate in order to promote maximum conductance between conductors while limiting resistance from the tin-copper metallic bond. Tin is used as a surface layer since it is substantially softer than copper and may be recurrently wiped to provide a fresh de-oxidized surface for metal-to-metal connection between conductors.
Electrical contacts have been traditionally made by electroplating a layer of tin to copper substrates followed by stamping out individual conductors. The copper substrates must be cleaned prior to placement in the electroplating bath to remove any oxidized surface layers that may otherwise create additional electrical resistance. The substrates are coated to a thickness of about 3 to 5 microns of tin.
Because most electrical contacts undergo repeated connections and reconnections, increasing the thickness of the tin surface layer correlates well with the longevity and durability of the contact. However, due to processing limitations and increased frictional properties, the threshold thickness for electroplating tin onto copper is about 5 microns.
While it may be possible to use other available coating methods to increase coating thickness, methods that rely on melting and/or depositing the tin in a molten state are undesirable because, unless conducted in the absence of oxygen, they will introduce significant oxidation into the tin surface layer. Also, due to the increased costs of use, such methods are not practical.
One of the main problems with present electrical contacts is debris build-up due to fretting on the contact surface. With relative movement of mated electrical contacts, a small portion of the oxidized surface layer is rubbed away to expose a fresh electrical connection surface. The portion rubbed away usually does not flake off, but instead remains adjacent to the contact point and begins to create a build-up of oxidized debris. It is well known that this oxidized debris becomes a source for additional resistance and degradation of the contact's conductance.
Prior to the present invention, removal of this debris has been impractical. In the prior art, the solution has been to provide continuous layer coatings that have been believed to result in maximum surface area for conductance.
A new technique for producing coatings by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62–71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream and projecting them against a target substrate. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate.
This work improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.
The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov's apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns and up to about 106 microns.
This modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings is disclosed in U.S. Pat. No. 6,139,913, Van Steenkiste et al., that issued on Oct. 31, 2000. The process and apparatus provide for heating a high pressure air flow up to about 650° C. and accelerating it with entrained particles through a de Laval-type nozzle to an exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are sufficiently lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, no phase transition occurs in the particles prior to impingement.